Several studies are being conducted using transcranial magnetic stimulation to examine if a) there is bilateral representation of the laryngeal musculature in each cortex; b) if responses to stimulation on the right or left cortices are equivalent in amplitude and latency; and c) if laryngeal muscle responses are changed depending upon whether the activation task is respiration, speech or effort closure. In a previous study, stimulation on the left side produced increased amplitudes and reduced response latencies on both sides of the larynx in comparison with stimulation on the right. A second study determined the thresholds for contralateral muscle responses to cortical stimulation on the left and right sides. No significant differences were found between the threshold values on the two sides in 4 subjects for either the laryngeal or hand muscles. When stimuli were presented at threshold levels of 10% greater, two types of responses occurred: those that were within 10 ms after stimulation and those that were between 10 and 20 ms. The early responses were found to be greater in amplitude and reduced in latency on the ipsilateral side. A fourth study was conducted aimed at determining whether stimulation of the nerve at the periphery at the point where it emerges from the skull, produced responses similar to the early ipsilateral responses. When the same normal volunteers were stimulated both peripherally and at the cortex, the peripheral nerve responses were similar to the early cortical responses with only a 1-2 Ms delay, indicating that early ipsilateral responses seen during transcranial magnetic stimulation are most likely due to direct stimulation of the peripheral nerve. We plan to use this technique to examine for differences from normal in central laryngeal neural control in patients with vocal fold movement disorders.
